Due to wide use of mobile terminals such as cellular phones, laptop personal computers and the like, the importance of batteries as a power source has gained recognition. These batteries are required to be compact, lightweight, and large-capacity, and further required capability hard to deteriorate despite repeated charge and discharge.
From a viewpoint of high-energy density and lightweight, lithium metal is sometimes used for a negative electrode, but in this case, phenomena of deposition of capillary crystal (dendrite) on lithium surface or of peeling of this dendrite from a collector occur in association with progress of the charge and discharge cycles. As a result, the dendrite penetrates a separator to cause short circuit inside, whereby there have been problems that life of a battery shortens and that cycle property deteriorates.
Thus, carbon material free from the above-mentioned problems is used for practical batteries. A representative example is the carbon material of graphite base, however, the amount of lithium ion that the material can occlude is restricted by an amount insertable between layers of graphite, therefore it is difficult to excess 372 mAh/g of the specific capacity. A method using an oxide capable of occluding lithium ion more than graphite and being large in the specific capacity has been developed.
For instance, Japanese Laid-Open Patent Publication No. 2000-12036 proposes a method that metal fine particles are added and dispersed in melted oxide particles to enhance the electron conductivity, thereby effectively using lithium ion occluding site.
Japanese Laid-Open Patent Publication No. H10-334889 also discloses a negative electrode in which carbon particles supporting two or more kinds of metals including metals such as Ag and Sn that forms alloy with lithium and metals such as Cu that do not form alloy with lithium are made into a mass of the above-mentioned particles with use of a bonding agent and the mass is used as a negative electrode active material.
In the above-mentioned prior art, however, it is difficult to obtain sufficient capacity.
For instance, in an art described in Japanese Laid-Open Patent Publication No. H10-334889, a negative electrode with a configuration in which a plurality of metals are supported on carbon particles has been problems that the initial discharge capacity is lower than a capacity expected from used material and further a capacity is remarkably reduced after the charge and discharge cycles. These problems will be described below.
The above-mentioned reduction in the initial discharge capacity is considered to result from peeling of the bound portion among particles and damage on an electrode active material layer in an initial charge and discharge process. Material such as Sn that is large in an amount of lithium occlusion has a large charge and discharge capacity, but variation in volume in association with charge and discharge is large. Therefore, when carbon particles supporting such a material is bound to configure a negative electrode, it is considered that in the initial charge and discharge process, a part of the bound portion among particles is damaged, whereby a rise in internal resistance and a reduction in capacity occur.
On the other hand, the reduction in capacity after the charge and discharge cycles is inferred to be caused by not only damage on the bound portion among the particles in association with the above-mentioned variation in volume, but also microscopic inhomogeneity in an electric field occurred in the negative electrode. When metal particles and the like is mixed in the carbon particles, nonuniformity in distribution usually arises due to difference and so on in properties of the powder. Carbon particles and metal particles have different resistivity and specific capacity respectively, therefore the microscopic inhomogeneity in distribution of the electric field occurs. Thus, since the variation in volume in the charge and discharge process is localized, the configuration of the negative electrode is broken down, consequently the reduction in capacity occurs.